1. Field of the Invention
The present invention relates to viscosifying agents and, more specifically, to viscosifying agents useful in forming aqueous-based viscoelastic fluids and methods of using such agents.
2. Description of the Related Art
The rheological properties of fluids used in a broad range of industrial and consumer-oriented applications are routinely modified in order to improve the performance of such fluids. For example, many types of fluids must be thickened such that the fluids have a viscosity sufficient for the particular application for which the fluid is intended for use. Viscosifying agents are typically used to impart the required viscosity to such fluids.
Oilfield, industrial and consumer product fluids are representative of the types of fluids to which viscosifying agents are added to enhance the viscosity and performance of such fluids. Oilfield fluids which require enhanced viscosities may include fluids for use in hydraulic fracturing, drilling, well completion and gravel packing operations. Thickened fluids may also be used in industrial applications such as for use as drag reducing agents for pipeline transport of fluids. Consumer product fluids which commonly incorporate viscosifying agents may include shampoos, hard surface cleaners, detergents and the like.
For oilfield fluids, highly viscous fluids facilitate many different aspects of the drilling process. For example, hydraulic fracturing operations utilize viscous fluids to stimulate the recovery of fluids, such as oil, natural gas or brine, from a reservoir located in a subterranean formation. In the process, a fracturing fluid is pumped downhole into a wellbore at pressures sufficient to fracture the subterranean formation. Proppant material (e.g., sand, glass beads, polystyrene beads, etc.) is suspended in the fracturing fluid and is thereby carried into the newly developed fractures and fissures in the subterranean formation. When the pressure is released from the fracturing fluid, the proppant serves to keep the subterranean formation from closing back on itself. Beneficially, the formation fractures stabilized by the deposited proppant are permeable to liquid and gas flow from the reservoir. Because of this formation permeability, oil, natural gas, brine or other subterranean fluids are able to flow into the wellbore for recovery. The fracturing operation is considered an enhanced oil recovery (EOR) technique designed to increase fluid recovery.
Hydraulic fracturing fluids are thickened or viscosified mainly to initiate and propagate fractures of the subterranean formation and to suspend the proppant. The fracturing fluids are typically aqueous-based and may be composed of fresh water or brine. Historically, polymers such as naturally occurring polysaccharides and modified cellulosic derivatives have been used to viscosify fracturing fluids. The polymers are frequently cross-linked with organometallic compounds (borates, zirconates, titanates, etc.) to achieve highly viscous fluids.
There are a number of disadvantages associated with fracturing fluids thickened with polymer-based materials. One disadvantage of polymer-based fluids is that fluid viscosity is sensitive to shear. A further disadvantage of polymer-based fluids is that a chemical breaker, such as a peroxide, must be utilized if the user wishes to decrease the viscosity of the fluid. Preparation of the polymer-based fluids is time consuming and generally requires expensive equipment at the wellsite.
Furthermore, polymer-based fracturing fluids are known to cause wellbore damage through precipitation of residual chemical fragments or undisolved material. The polysaccharide-based systems usually contain insoluble materials that can be difficult to remove from the formation resulting in residues that seal or plug the created fractures and fissures. Similarly, the modified cellulose derivatives can form inconsistent fluids due to incomplete hydration. The insoluble cellulosic materials can cause wellbore damage by precipitating out of solution onto the formation face. For maximum reservoir fluid recovery, it is essential that all chemical residues are removed from the wellbore following a hydraulic fracturing operation.
Viscoelastic surfactant-based fluids represent a group of fluids which may have utility in oilfield applications, such as fracturing operations. And, viscoelastic fluids offer benefits not found in polymer-based fluids. Viscoelastic fluids are fluids which exhibit both solid-like (elastic) and liquid-like (viscous) character. Surfactant-based aqueous systems are referred to as viscoelastic if they at least partially recover to their initial state after abatement of an applied stress. For such viscoelastic surfactant-based systems, it is thought that under certain conditions (usually in the presence of brine) cationic, amphoteric and zwitterionic surfactants associate into worm-like, rod-like or cylindrical micelles which entangle much like polymers to produce viscosity. It is thought that electrostatic intermolecular interactions between the surfactant molecules are responsible for the micelle formation. Thus, the micelles are disrupted by application of shear forces and reform once the shear forces are abated. This differs from polymer-based systems which involve covalent bonds that may be permanently broken under shear resulting in loss of fluid viscosity.
One qualitative test useful for determining the characteristic of viscoelasticity is to determine whether the formulation exhibits properties referred to as “recoil” or “rebound.” To determine viscoelasticity based on recoil, a formulation is simply swirled in a manner that creates suspended bubbles. If recoil is observed when the swirling motion is stopped, the solution is considered to be viscoelastic. This characteristic is a function of spatial memory associated with the elastic or solid-like nature of a viscoelastic fluid.
Viscoelastic surfactants may also be characterized by reversible shear thinning. When such a fluid is passed through a high shear environment, such as a pump, low viscosity is observed. When the shear is removed, the rod-like micelles are reformed and higher viscosity is restored.
Therefore, viscoelastic fluids would appear to be potentially quite advantageous in outfield applications and other applications because they can be pumped easily at low viscosity yet reform as a high viscosity fluid following pumping. Such viscoelastic fluids may be further advantageous for oilfield applications because they generally lose viscosity on dilution in water-miscible hydrocarbons, such as lower molecular weight components of reservoir fluid. This characteristic would greatly reduce wellbore damage from chemical precipitation.
Viscoelastic fluids, however, are potentially temperature sensitive. Kaler in Highly Viscoelastic Wormlike Micellar Solutions Formed by Cationic Surfactants with Long Unsaturated Tails, Langmuir, 2001, 17, 300–306 indicates that it is commonly theorized that as temperature increases, the length of the rod-like micelles decreases. As such, the viscosity due to micelle entanglement is decreased with increased temperature. This is potentially disadvantageous in outfield applications where elevated temperatures are encountered.
A number of viscoelastic fluids are identified in the literature. However, such fluids are not entirely satisfactory, for example, because they may be unstable under conditions typically encountered in oilfield applications. For example, U.S. Pat. No. 5,551,516 (Norman et al.) discloses the use of quaternary ammonium salts in conjunction with an inorganic salt and a stabilizing organic salt to generate aqueous-based viscoelastic fluids. These compounds are described to be useful as fracturing fluids, but are limited by high temperature instability. Similarly, U.S. Pat. No. 6,239,183 (Farmer et al.) discloses the use of amidoamine oxides, alkoxylated monoamine salts of aromatic dicarboxylic acids and alkyldiamine salts of aromatic dicarboxylic acids as viscoelastic surfactants. These compounds are also claimed to be effective as fracturing fluids but are limited by high temperature instability. When the aforementioned fluids encounter high temperatures, viscosity is lost which causes the proppant to prematurely settle into the formation and additionally causes the fracturing fluid to leak off into the formation.
Oilfield applications, other than fracturing operations, also require the use of thickened fluids. For example, drilling fluids or muds are circulated through the drill bit and up the wellbore annulus during drilling operations. It is necessary for these fluids to maintain high viscosity and density. High viscosity is necessary to transfer drill cuttings up the annulus for separation at the surface and to transfer energy to the drill bit. The fluids must have high density to control pressure on the formation and to support the weight of the drill string. Although drilling muds have traditionally been thickened with clays (bentonite, attapulgite, etc.), U.S. Pat. No. 6,426,321 (Durrieu, et al.) discloses the use of surfactant mixtures to produce biodegradable drilling muds.
Thickened fluids are also required as so-called “completion fluids.” After a well has been drilled, it is necessary to flush out the wellbore prior to casing. This is accomplished with a completion fluid. These fluids are typically high brine solutions that are viscosified to avoid fluid leak-off into the formation. The completion fluid further serves to avoid premature reservoir fluid entry into the wellbore during perforation. The thickening agent must be capable of thickening the completion fluid under the high-brine conditions typically encountered immediately following the drilling operation.
Yet another oilfield application requiring thickened fluids involves “gravel packing” operations. Gravel packing operations are used to avoid excess sand production during reservoir fluid recovery. In this operation, after sand clean-out, gravel suspended in a thickened fluid is pumped into the sand producing zone. Once the gravel is in place, a wire-wrapped screen, or screen liner, is positioned between the production tubing and the gravel. The sand is then prevented from entering the production tubing by gravel filtration. As discussed by Nehmer in “Viscoelastic Gravel-Pack Carrier Fluid” Society of Petroleum Engineers (SPE) Paper 17168, surfactants find utility as viscosifying agents in gravel packing operations.
Thickening agents are also used in industrial applications as “drag reducing” agents. Thickening agents in the form of surfactants may be added to an aqueous solution that is stirring at a high rate. The vortex is quickly eliminated during dissolution of the surfactant. Drag reducing agents find use in pipeline transport of fluids to reduce turbulent flow. The utility of cationic surfactants as drag reducing agents is discussed by Campbell and Jovancicevic in “Performance Improvements from Chemical Drag Reducers” Society of Petroleum Engineers (SPE) Paper 65021.
The use of viscoelastic surfactants in consumer product fluids is exemplified by U.S. Pat. No. 5,639,722 (Kong et al.), wherein acidic cleaning compositions are thickened with cetyl trimethylammonium chloride and sodium xylene sulfonate. In U.S. Pat. No. 5,833,764 (Rader et al.) the use of cetyl trimethylammonium chloride as a viscosifier for an aqueous-based drain opener is disclosed.
It would be an improvement in the art to provide a viscoelastic thickening fluid and method of use of such fluid which would be capable of thickening fluids used in many different applications, which would thicken fluids under temperature and environmental conditions typically encountered in the particular application and which would provide an alternative to existing thickening agents.